Typical mass spectrometers contain an ion source having an ionization chamber. A sample containing an analyte is introduced into the ionization chamber through a means for sample introduction. Once the analyte is disposed within the ionization chamber, an ionization source produces ions from the sample. The resultant ions are then processed by at least one analyzer or filter that separates the ions according to their mass-to-charge ratio. The ions are collected in a detector, which measures the number and distribution of the ions, and a data processing system uses the measurements from the detector to produce the mass spectrum of the analyte. The sample can be in gaseous form or, depending upon the particular analyte separation and ionization means, can initially be a component of a liquid or gel.
There are many types of ion sources that are useful in mass spectrometry (hereinafter referred to as MS). Sources of ionization sources include, but are not limited to, electron impact, chemical ionization, plasma, fast ion or atom bombardment, field desorption, laser desorption, plasma desorption, thermospray and electrospray. Two of the most widely used sources for gaseous analytes are the electron impact (hereinafter referred to as EI) and chemical ionization (hereinafter referred to as CI) sources.
An EI source generally contains a heated filament giving off electrons that are accelerated toward an anode and collide with gaseous analyte molecules introduced into the ionization chamber. Typically, the electrons have energies of about 70 eV and produce ions with an efficiency of less than a few percent. This energy is typically chosen because it is well in excess of the minimum energy required to ionize and fragment molecules and is at or near the peak of the ionization efficiency curve for most molecules. The total pressure within the ionization source is normally held at less than about 10−3 torr. The ions produced are extracted from the EI source with an applied electric field and introduced into an analyzer wherein they are separated by mass-to-charge ratio. The selected ions are registered as an ion current characteristic of the specified mass/charge by the ion detection and signal processing system of the mass spectrometer. Those ions ideally do not collide with other molecules or surfaces from the time they are formed in the EI source until the time they are collected in the detector. An EI source is often employed in MS in conjunction with gas chromatography (GC), which separates constituents of the analyte by time of elution.
The EI ion source is often used with a quadrupole mass spectrometer for reasons of stability and reproducibility of ion-fragmentation patterns. The patterns produced can be compared with “classical” spectra libraries and the ion's molecular composition thereby determined. Thus the quality of the spectral pattern produced by the ion source may greatly effect the interpretation of data.
In EI, the character and quantity of analyzable ions from the molecules in the sample depend upon reactions occurring on the inner surfaces of the chamber containing the source of ionization. First, the analyte is introduced into an ionization chamber wherein ionization of the analyte is intended. Before ionization, however, much of the sample is exposed to inner surfaces of the chamber, which are usually heated. The interaction of the sample with these surfaces may create an undesired effect. For example, if a portion of the sample adheres to the chamber surface, the portion cannot be effectively ionized and directed to the detector. As a result, the sensitivity of the apparatus for analysis of that analyte may suffer. In addition, the sample can degrade, i.e., convert to other compounds or be adsorbed onto the surface of the chamber and desorb later. Depending upon the compound, many unexpected ions can appear as a result of the interaction of the compound with the surfaces. The results are undesirable: chromatographic peak tailing, loss of sensitivity, nonlinearity, erratic performance and the like.
In addition, cleanliness is critical to the proper performance of the mass spectrometer using an EI source, particularly for quantitative analysis of material in a low concentration, such as for GC/MS analysis of pesticide residues, drug residues and metabolites, environmental samples and trace analysis of organic compounds. The relatively non-volatile materials in the sample matrix generally form insulating deposits on the surface of the chamber that take on an electrical charge. This charge distorts the applied electric field causing anomalies in ion production. Often, abrasive cleaning is employed to ensure that the chamber is substantially free of insulating deposits.
In contrast to the EI ion source, a CI source produces ions through collision of the molecules in the analyte with primary ions present in the ionization chamber or by attachment of low energy electrons present in the chamber. A CI source operates at much higher pressures than an EI source in order to permit frequent collisions. The overall pressure in a CI source during operation typically ranges from about 0.1 to about 2 torr. This pressure may be produced by the flow of a reagent gas, such as methane, isobutane, ammonia or the like, that is pumped into the chamber containing the CI source. In a typical configuration, both the reagent gas and the analyte are introduced through gas-tight seals into the chamber containing the CI source. The reagent gas and the analyte are sprayed with electrons having energies of 50 to 300 eV from a filament through a small orifice, generally less than 1 mm in diameter. Ions formed are extracted through another small orifice, also generally less than 1 mm in diameter, and introduced into the analyzer or filter. Electric fields may be applied inside the CI source, but they are usually not necessary for operation of the CI source. Ions eventually leave the CI source through a combination of diffusion and entrainment in the flow of the reagent gas. Thus, it is evident that CI sources operate in a substantially different manner from EI sources. However, the same undesired interactions of the sample with the source chamber surfaces may occur in a CI source as in an EI source as mentioned above.
Efforts have been made to address sample degradation problems in the ionization chamber of a mass spectrometer, particularly those containing an EI ion source, by substituting for or modifying the surfaces of the ionization chamber. Such efforts include providing a metallic surface with advantageous properties. For example, ionization chambers have been made with electropolished stainless steel surfaces in efforts to reduce the total active surface area. However, mass spectrometers using such ionization chambers have been found to give variable results and still exhibit degradation of the analyte over time. U.S. Pat. No. 5,055,678 to Taylor et al. describes the use of a chromium or oxidized chromium surface in a sample analyzing and ionizing apparatus, such as an ion trap or EI ionization chamber, to prevent degradation or decomposition of a sample in contact with the surface. This reference also describes that coating the inner surface of the ionization chamber with materials known for corrosion resistance or inertness, such as gold, nickel and rhodium, may reduce degradation of analytes, such as pesticides, drugs and metabolites, to some degree. Such surfaces suffer from a variety of drawbacks such as susceptibility to scratching when the metal coating is soft or assembly/disassembly difficulties when the coating has a high coefficient of friction.
In addition, U.S. Pat. No. 5,633,497 to Brittain et al. describes the use of a thin coating of an inert, inorganic non-metallic insulator or semiconductor material on the interior surfaces of an ion trap or EI ionization chamber to reduce adsorption, degradation or decomposition of a sample contacting the chamber surface. The material disclosed in this reference was fused silica, with aluminum oxide, silicon nitride and “selected semiconductors” given as alternative embodiments. Because these surface coatings exhibit high electrical resistivity, however, electrical charge can undesirably accumulate on these coatings if the coatings are too thick. The important feature of the invention described in this reference is the use of a sufficiently thin coating of insulator that charging effects do not occur.
U.S. Pat. No. 5,796,100 to Palermo discloses a quadrupole ion trap having inner surfaces formed from molybdenum.
In addition, U.S. Pat. No. 6,037,587 to Dowell et al. describes a mass spectrometer having a CI source containing a chemical ionization chamber having inner surfaces formed from molybdenum.
Others have attempted to prevent degradation problems by treating the inner metal surfaces of the analytical apparatus with a passivating agent to mask or destroy active surface sites. For example, alkylchlorosilanes and other silanizing agents have been used to treat injectors, chromatographic columns, transfer lines and detectors in GC. See, e.g., U.S. Pat. No. 4,999,162 to Wells et al. Such treatments have been successful in deactivating metal surfaces and thus have prevented degradation of some species of analyte. Unfortunately, the materials used for such treatments have a sufficiently high vapor pressure to introduce organic materials in the gas phase within the volume of the ionization chamber that are ionized along with the analyte, producing a high chemical background in the mass spectrum.
One problem with the described techniques is that each of the metals is applied as a coating. Over time, the coatings will begin to wear or tarnish. It, therefore, would be desirable to be able to construct an ionization chamber for reactive samples that employs a single metal or alloy. This provides for longer wear as well as makes the manufacture of the parts less complicated.
To date, there are very few patents or inventions related to the use of “super alloys” with mass spectrometer parts. For instance, U.S. Pat. No. 6,025,591, discloses the use of a nickel coating on glass fibers to make conducting surfaces for the quadrupole of a mass spectrometer. Furthermore, U.S. Pat. No. 5,625,185 claims the use of graphite for entrance lenses of the ion optics of an ICP-MS system to avoid sputtering effects seen when using normal steels containing iron, nickel, and chromium.
Thus, there is a need to reduce the adsorption, degradation and decomposition of these important analyte ions in an ionization chamber and to mitigate the problems associated with known coatings.